Formation of AlO, AlOH, and Al(OH)₃ in the Interstellar Medium and Circumstellar Envelopes of AGB Stars

Abstract

AlO, AlOH, and Al(OH)3 can be formed in the gas-phase starting from nothing more than simple aluminum hydrides (AlH and AlH2) and water molecules. All three products are probable precursors to aluminum oxide clusters that may initiate the nucleation of dust grains in the interstellar medium. Chemically accurate CCSD(T)-F12b/cc-pVTZ-F12 computations provide exothermic energetic values for these pathways. For example, the fully submerged formation of AlO is exothermic by 51.3 kcal mol–1, and this should also lead to favorable kinetics. To aid in the detection of the recurring intermediate cis/trans-HAlOH with instrumentation located on the James Webb Space Telescope, among other observatories, rotational and vibrational spectroscopic data are reported by utilizing a highly accurate quartic force field methodology. The ν2 stretching frequency at 1807.9 cm–1 exhibits an anharmonic intensity of 185 km mol–1 and the antisymmetric bend (ν4) at 534.2 cm–1 exhibits an anharmonic intensity of 213 km mol–1 for the cis and trans isomers, respectively. These are roughly three times the antisymmetric stretch intensity of water. The cis isomer has a smaller dipole moment of 0.83 D, while the trans isomer contains a moderate dipole moment of 1.49 D. These properties indicate that both of these isomers can be observed through vibrational and rotational spectroscopic techniques.

A Possible Additional Formation Pathway for the Interstellar Diatomic SiS

“…Additionally, Figure 1 also explores the reaction of H 2 S with SiH 2 in spite of the lack of SiH 2 observation. While this process forms a stable intermediate (H 2 S-SiH 2 ) resulting from a dative bond between a lone pair on the sulfur and the empty p orbital on the silicon, as shown in related work (Grosselin & Fortenberry 2022;Flint & Fortenberry 2023;Palmer & Fortenberry 2024;Firth et al 2024), the creation of TS1 where an H 2 molecule is prepared for departure from the system is barely exothermic, lying −0.9 kcal mol −1 below the pair of dihydride starting materials. While this would once more produce H 2 , the effective "solvent of the Universe" (Woon 2023; Fortenberry 2024), the barrier is likely too close to the reactants for any population to traverse this saddle.…”

“…Additionally, the question is also open if hydrogen sulfide (H 2 S) or SH can replace water or the hydroxyl radical in similar reactions. AlO can form from AlH + OH (Firth et al 2024), and such leads to the suggestion that radical-radical reactions extrapolated to SiH + SH may proceed to SiS.…”

“…Recent work has shown that isolated molecule reaction pathways starting from water and metal hydrides can lead to both simple, diatomic metal oxides as well as larger clusters of inorganic oxides (Grosselin & Fortenberry 2022;Flint & Fortenberry 2023;Firth et al 2024); a similar case is also present for aluminum hydride plus ammonia (Palmer & Fortenberry 2024). These reactions exhibit barrierless entrance intermediates, showcase submerged reaction barriers, and generate ubiquitous H 2 as a stabilizing leaving group.…”